Software-Defined Networking (SDN) integration to WiFi introduces various benefits including flow-based traffic steering, intelligent access point (AP) association, and etc. However, scalability and reliability are the critical issues on the use of SDN for large-scale WiFi deployment, where, for example, mobile devices actively move around in urban areas. While Multidomain SDN (MSDN) is considered as a promising solution to such issues, the mobility management under MSDN environment has not been scrutinized well. Our goal is to provide mobile users with uninterrupted communication with the minimal network overhead. This paper proposes a system called LEGION that addresses the problems of mobility management in MSDN: 1) Location Discovery and Management, and 2) Flow Redirection. We implement the prototype of LEGION to validate its responsiveness, scalability, and effectiveness. Furthermore, compared with the other approaches, Broadcasting and Centralized Location Server, LEGION exhibits the optimum space complexity while utilizing the constant number of messages. © 2018 IEEE.